Semiconductor device and electronic device including the semiconductor device

ABSTRACT

A semiconductor device with favorable electrical characteristics is provided. Alternatively, a semiconductor device with a high on-state current is provided. Alternatively, a semiconductor device that is suitable for miniaturization is provided. A semiconductor device includes an oxide semiconductor, an insulating film, a gate insulating film, and a gate electrode. The oxide semiconductor includes a first portion and a second portion over the first portion. The insulating film includes a region in contact with a side surface of the first portion. The gate electrode includes a region that covers the second portion with the gate insulating film provided therebetween.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transistor and a semiconductor device, and a manufacturing method thereof, for example. The present invention relates to, for example, a display device, a light-emitting device, a lighting device, a power storage device, a memory device, an imaging device, a processor, and an electronic device. The present invention relates to a method for manufacturing a display device, a liquid crystal display device, a light-emitting device, a memory device, an imaging device, and an electronic device. The present invention relates to a driving method of a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a memory device, an imaging device or an electronic device.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter.

In this specification and the like, a semiconductor device generally means a device that can function by utilizing semiconductor characteristics. A transistor and a semiconductor circuit are embodiments of semiconductor devices. In some cases, a storage device, a display device, or an electronic device includes a semiconductor device.

2. Description of the Related Art

In recent years, a technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) or an image display device (also simply referred to as a display device). As semiconductor materials applicable to the transistor, silicon-based semiconductor materials have been widely used, but oxide semiconductors have been attracting attention as alternative materials.

For example, a technique for forming a transistor using zinc oxide or an In—Ga—Zn oxide semiconductor as an oxide semiconductor is disclosed (see Patent Documents 1 and 2).

Demand for an integrated circuit in which semiconductor elements such as a miniaturized transistor are integrated with high density has risen with increased performance and reductions in the size and weight of an electronic device.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.     2007-123861 -   [Patent Document 2] Japanese Published Patent Application No.     2007-096055

SUMMARY OF THE INVENTION

Miniaturization of transistors has been progressing with an increase in the degree of integration of circuits. In some cases, miniaturization of transistors causes deterioration of the electrical characteristics of the transistors, such as on-state current, off-state current, threshold voltage, and a subthreshold swing value (an S value). In general, a decrease in channel length leads to an increase in off-state current, an increase in variations of threshold voltage, and an increase in S value. When the channel width is decreased, the on-state current is reduced.

An object of one embodiment of the present invention is to provide a semiconductor device with favorable electrical characteristics. Another object of one embodiment of the present invention is to provide a semiconductor device with high on-state current. Another object is to provide a semiconductor device that is suitable for miniaturization. Another object is to provide a highly integrated semiconductor device. Another object of one embodiment of the present invention is to provide a semiconductor device with low power consumption. Another object of one embodiment of the present invention is to provide a highly reliable semiconductor device. Another object is to provide a semiconductor device which can retain data even when power supply is stopped. Another object is to provide a novel semiconductor device.

Note that the descriptions of these objects do not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Other objects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

One embodiment of the present invention is a semiconductor device including an oxide semiconductor, an insulating film, a gate insulating film, and a gate electrode. The oxide semiconductor includes a first portion and a second portion over the first portion. The insulating film includes a region in contact with a side surface of the first portion. The gate electrode includes a region that covers the second portion with the gate insulating film provided therebetween.

In one embodiment of the present invention, the second portion may be provided above the insulating film in a film thickness direction of the insulating film.

In one embodiment of the present invention, the oxide semiconductor may contain at least one element selected from indium (In), gallium (Ga), and zinc (Zn).

In one embodiment of the present invention, the insulating film may have a function of releasing oxygen by heating.

One embodiment of the present invention is an electronic device including the above-described semiconductor device, and a display device or a battery.

Note that in the semiconductor device of one embodiment of the present invention, the oxide semiconductor may be replaced with another semiconductor.

In one embodiment of the present invention, any of the following semiconductor devices can be provided: a semiconductor device with favorable electrical characteristics, a semiconductor device that is suitable for miniaturization, a semiconductor device with high on-state current, a highly integrated semiconductor device, a semiconductor device with low power consumption, a highly reliable semiconductor device, a semiconductor device which can retain data even when power supply is stopped, and a novel semiconductor device.

Note that the descriptions of these effects do not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the above effects. Other effects will be apparent from and can be derived from the descriptions of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor.

FIGS. 2A to 2C illustrate a method for manufacturing a transistor.

FIGS. 3A to 3C illustrate a method for manufacturing a transistor.

FIGS. 4A and 4B illustrate a method for manufacturing a transistor.

FIGS. 5A and 5B are cross-sectional views of a transistor.

FIGS. 6A to 6C are cross-sectional views of a transistor.

FIGS. 7A and 7D are cross-sectional views of semiconductor devices and FIGS. 7B and 7C are circuit diagrams of semiconductor devices.

FIGS. 8A and 8C are circuit diagrams of a memory device and FIG. 8B is a cross-sectional view of a memory device.

FIG. 9 illustrates a configuration example of an RFID tag.

FIG. 10 illustrates a configuration example of a CPU.

FIG. 11 is a circuit diagram of a memory element.

FIG. 12A illustrates a configuration example of a display device and FIGS. 12B and 12C are circuit diagrams of pixels.

FIG. 13 illustrates a display module.

FIGS. 14A to 14F each illustrate an electronic device.

FIGS. 15A to 15F each illustrate an application example of an RFID.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention are described in detail with the reference to the drawings. However, the present invention is not limited to the description below, and it is easily understood by those skilled in the art that modes and details disclosed herein can be modified in various ways. Furthermore, the present invention is not construed as being limited to description of the embodiments. In describing structures of the present invention with reference to the drawings, common reference numerals are used for the same portions in different drawings. Note that the same hatched pattern is applied to similar parts, and the similar parts are not especially denoted by reference numerals in some cases.

Note that the size, the thickness of films (layers), or regions in drawings is sometimes exaggerated for simplicity.

In this specification, for example, when the shape of an object is described with use of a term such as “diameter”, “grain size (diameter)”, “dimension”, “size”, or “width”, the term can be regarded as the length of one side of a minimal cube where the object fits, or an equivalent circle diameter of a cross section of the object. The term “equivalent circle diameter of a cross section of the object” refers to the diameter of a perfect circle having the same area as that of the cross section of the object.

In this specification, a voltage usually refers to a potential difference between a given potential and a reference potential (e.g., a source potential or a ground potential (GND)). A voltage can be referred to as a potential and vice versa.

Note that the ordinal numbers such as “first” and “second” in this specification are used for convenience and do not denote the order of steps or the stacking order of layers. Therefore, for example, the term “first” can be replaced with the term “second”, “third”, or the like as appropriate. In addition, the ordinal numbers in this specification and the like are not necessarily the same as those which specify one embodiment of the present invention.

In this specification, a “semiconductor (or semiconductor film)” includes characteristics of an “insulator (or insulating film)” in some cases when the conductivity is sufficiently low, for example. Further, a “semiconductor” and an “insulator” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “insulator” is not clear. Accordingly, a “semiconductor” in this specification can be called an “insulator” in some cases. Similarly, an “insulator” in this specification can be called a “semiconductor” in some cases.

Note that a “semiconductor (or semiconductor film)” includes characteristics of a “conductor (or conductive film)” in some cases when the conductivity is sufficiently high, for example. Further, a “semiconductor” and a “conductor” cannot be strictly distinguished from each other in some cases because a border between the “semiconductor” and the “conductor” is not clear. Accordingly, a “semiconductor” in this specification can be called a “conductor” in some cases. Similarly, a “conductor” in this specification can be called a “semiconductor” in some cases.

In this specification, an impurity in a semiconductor refers to, for example, elements other than the main components of a semiconductor. For example, an element with a concentration of lower than 0.1 atomic % is an impurity. When an impurity is contained, the density of states (DOS) may be formed in a semiconductor, the carrier mobility may be decreased, or the crystallinity may be decreased, for example. In the case where the semiconductor is an oxide semiconductor, examples of an impurity which changes the characteristics of the semiconductor include Group 1 elements, Group 2 elements, Group 14 elements, Group 15 elements, and transition metals other than the main components; specific examples are hydrogen (included in water), lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen. Further, in the case where the semiconductor is a silicon film, examples of an impurity which changes the characteristics of the semiconductor include oxygen, Group 1 elements, Group 2 elements, Group 13 elements, and Group 15 elements.

In this specification, the phrase “A has a region with a concentration B” includes, for example, “the concentration of the entire region in a region of A in the depth direction is B”, “the average concentration in a region of A in the depth direction is B”, “the median value of a concentration in a region of A in the depth direction is B”, “the maximum value of a concentration in a region of A in the depth direction is B”, “the minimum value of a concentration in a region of A in the depth direction is B”, “a convergence value of a concentration in a region of A in the depth direction is B”, and “a concentration in a region of A in which a probable value is obtained in measurement is B”.

In this specification, the phrase “A has a region with a size B, a length B, a thickness B, a width B, or a distance B” includes, for example, “the size, the length, the thickness, the width, or the distance of the entire region in a region of A is B”, “the average value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the median value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the maximum value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “the minimum value of the size, the length, the thickness, the width, or the distance of a region of A is B”, “a convergence value of the size, the length, the thickness, the width, or the distance of a region of A is B”, and “the size, the length, the thickness, the width, or the distance of a region of A in which a probable value is obtained in measurement is B”.

Note that the channel length refers to, for example, a distance between a source (a source region or a source electrode) and a drain (a drain region or a drain electrode) in a region where a semiconductor and a gate electrode overlap with each other, a region where a current flows in a semiconductor when a transistor is on, or a channel formation region in a top view of the transistor. In one transistor, channel lengths in all regions are not necessarily the same. In other words, the channel length of one transistor is not limited to one value in some cases. Therefore, in this specification, the channel length is any one of values, the maximum values, the minimum value, or the average value in a channel formation region.

The channel width refers to, for example, the length of a portion where a source and a drain face each other in a region where a semiconductor and a gate electrode overlap with each other, a region where a current flows in a semiconductor when a transistor is on, or a channel formation region. In one transistor, channel widths in all regions do not necessarily have the same value. In other words, a channel width of one transistor is not fixed to one value in some cases. Therefore, in this specification, a channel width is any one of values, the maximum value, the minimum value, or the average value in a channel formation region.

Note that depending on transistor structures, a channel width in a channel formation region actually (hereinafter referred to as an effective channel width) is different from a channel width shown in a top view of a transistor (hereinafter referred to as an apparent channel width) in some cases. For example, in a transistor having a three-dimensional structure, an effective channel width is greater than an apparent channel width shown in a top view of the transistor, and its influence cannot be ignored in some cases. For example, in a miniaturized transistor having a three-dimensional structure, the proportion of a channel region formed in a side surface of a semiconductor is higher than the proportion of a channel region formed in a top surface of a semiconductor in some cases. In that case, an effective channel width obtained when a channel is actually formed is greater than an apparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effective channel width is difficult to measure in some cases. For example, to estimate an effective channel width from a design value, it is necessary to assume that the shape of a semiconductor is known as an assumption condition. Therefore, in the case where the shape of a semiconductor is not known accurately, it is difficult to measure an effective channel width accurately.

Therefore, in this specification, in a top view of a transistor, an apparent channel width that is a length of a portion where a source and a drain face each other in a region where a semiconductor and a gate electrode overlap with each other is referred to as a surrounded channel width (SCW) in some cases. Further, in this specification, in the case where the term “channel width” is simply used, it may denote a surrounded channel width and an apparent channel width. Alternatively, in this specification, in the case where the term “channel width” is simply used, it may denote an effective channel width in some cases. Note that the values of a channel length, a channel width, an effective channel width, an apparent channel width, a surrounded channel width, and the like can be determined by obtaining and analyzing a cross-sectional transmission electron microscope (TEM) image and the like.

Note that in the case where electric field mobility, a current value per channel width, and the like of a transistor are obtained by calculation, a surrounded channel width may be used for the calculation. In that case, a value might be different from one calculated by using an effective channel width.

Note that in this specification, the description “A has a shape such that an end portion extends beyond an end portion of B” may indicate, for example, the case where at least one of end portions of A is positioned on an outer side than at least one of end portions of B in a top view or a cross-sectional view. Thus, the description “A has a shape such that an end portion extends beyond an end portion of B” can be alternatively referred to as the description “one of end portions of A is positioned on an outer side than one of end portions of B”.

In this specification, the term “parallel” indicates that the angle formed between two straight lines is greater than or equal to −10° and less than or equal to 10°, and accordingly also includes the case where the angle is greater than or equal to −5° and less than or equal to 5°. The term “perpendicular” indicates that the angle formed between two straight lines ranges from 80° to 100°, and accordingly also includes the case where the angle ranges from 85° to 95°.

In this specification, trigonal and rhombohedral crystal systems are included in a hexagonal crystal system.

Note that in this specification, the terms “film” and “layer” can be interchanged depending on the case or circumstances. For example, the term “conductive layer” can be changed into the term “conductive film” in some cases. Also, the term “insulating film” can be changed into the term “insulating layer” in some cases.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of the present invention and a method for manufacturing the semiconductor device will be described with reference to drawings. As an example of a semiconductor device, a transistor will be described.

In a transistor of one embodiment of the present invention, silicon (including strained silicon), germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, an organic semiconductor, an oxide semiconductor, or the like can be used for a channel formation region. It is particularly preferable to use an oxide semiconductor having a wider band gap than silicon for the channel formation region.

The oxide semiconductor preferably contains at least indium (In) or zinc (Zn), for example. Alternatively, the oxide semiconductor contains an oxide represented by an In-M-Zn-based oxide (M is a metal such as Al, Ti, Ga, Ge, Y, Zr, Sn, La, Ce, or Hf).

In the description below, unless otherwise specified, a transistor described as an example includes an oxide semiconductor in a channel formation region.

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor 150 of a semiconductor device. FIG. 1A is a top view of the transistor 150. FIG. 1B is a cross-sectional view taken along dashed-dotted line A1-A2 in FIG. 1A. FIG. 1C is a cross-sectional view taken along dashed-dotted line B1-B2 in FIG. 1A. In FIGS. 1A to 1C, some components are enlarged, reduced in size, or omitted for easy understanding. In some cases, the direction of the dashed-dotted line A1-A2 is referred to as a channel length direction, and the direction of the dashed-dotted line B1-B2 is referred to as a channel width direction.

The transistor 150 illustrated in FIGS. 1A to 1C includes a base insulating film 102 over a substrate 100, an oxide semiconductor 104 over the base insulating film 102, an insulating film 106 over the base insulating film 102, a source electrode 108 a and a drain electrode 108 b in contact with the oxide semiconductor 104, an oxide semiconductor 110 in contact with the source electrode 108 a and the drain electrode 108 b, a gate insulating film 112 over the oxide semiconductor 110, a gate electrode 114 over the gate insulating film 112, and an insulating film 116 over the gate electrode 114. The oxide semiconductor 104 includes a first portion 104 a and a second portion 104 b over the first portion 104 a. The insulating film 106 has a region in contact with a side surface of the first portion 104 a.

In addition, the second portion 104 b is formed above the insulating film 106 in the thickness direction of the insulating film 106.

The base insulating film 102 may have a function of releasing oxygen by heating. Such an insulating film including oxygen (also referred to as excess oxygen) released by heating is provided in contact with the oxide semiconductor, whereby oxygen vacancies contained in the oxide semiconductor can be filled. Thus, the semiconductor device including the oxide semiconductor can have favorable electrical characteristics.

The transistor 150 illustrated in FIGS. 1A to 1C includes the base insulating film 102, but the insulating film 102 is not necessarily provided.

Note that at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided on at least part (or the whole) of a surface, a side surface, a top surface, and/or a bottom surface of a semiconductor film such as the oxide semiconductor 104.

Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is in contact with at least part (or the whole) of a surface, a side surface, and/or a bottom surface of a semiconductor film such as the oxide semiconductor 104. Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is in contact with at least part (or the whole) of a semiconductor film such as the oxide semiconductor 104.

Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is electrically connected to at least part (or the whole) of a surface, a side surface, and/or a bottom surface of a semiconductor film such as the oxide semiconductor 104. Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is electrically connected to part (or the whole) of a semiconductor film such as the oxide semiconductor 104.

Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided close to at least part (or the whole) of a surface, a side surface, and/or a bottom surface of a semiconductor film such as the oxide semiconductor 104. Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided close to part (or the whole) of a semiconductor film such as the oxide semiconductor 104.

Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided next to at least part (or the whole) of a surface, a side surface, and/or a bottom surface of a semiconductor film such as the oxide semiconductor 104. Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided next to part (or the whole) of a semiconductor film such as the oxide semiconductor 104.

Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided obliquely above at least part (or the whole) of a surface, a side surface, and/or a bottom surface of a semiconductor film such as the oxide semiconductor 104. Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided obliquely above part (or the whole) of a semiconductor film such as the oxide semiconductor 104.

Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided above at least part (or the whole) of a surface, a side surface, and/or a bottom surface of a semiconductor film such as the oxide semiconductor 104. Alternatively, at least part (or the whole) of the source electrode 108 a (and/or the drain electrode 108 b) is provided above part (or the whole) of a semiconductor film such as the oxide semiconductor 104.

The insulating film 106 may have a function of releasing oxygen by heating. Such an insulating film including oxygen (also referred to as excess oxygen) released by heating is provided in contact with the oxide semiconductor, whereby oxygen vacancies contained in the oxide semiconductor can be filled. Thus, the semiconductor device including the oxide semiconductor can have favorable electrical characteristics.

As illustrated in FIGS. 1A to 1C, in the case where a three-dimensional oxide semiconductor is used in an active layer of the transistor, oxygen can be efficiently supplied from the side surfaces of the oxide semiconductor 104 that are in contact with the insulating film 106 to the oxide semiconductor 104.

The insulating film 116 preferably serves as a barrier film that blocks oxygen, hydrogen, water, and the like. The insulating film 116 with such a function can prevent hydrogen and water from entering the oxide semiconductor 104 from the outside, and oxygen in the oxide semiconductor 104 from being released to the outside. Note that it is preferable that hydrogen, water, and the like in the insulating film 116 be reduced as much as possible. It is preferable that hydrogen, water, and the like be released as less as possible.

Furthermore, it is possible to prevent outward diffusion of oxygen from the oxide semiconductor and entry of hydrogen, water, and the like into the oxide semiconductor from the outside by providing an insulating film having a blocking effect against oxygen, hydrogen, water, and the like as the insulating film 116. The insulating film having a blocking effect against oxygen, hydrogen, water, and the like may be formed to have a single-layer structure or a stacked-layer structure using, for example, an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, or a hafnium oxynitride film.

Next, the details of other components of the transistor 150 will be described below.

There is no particular limitation on a material or the like of the substrate 100 as long as the material has heat resistance enough to withstand at least heat treatment to be performed later. For example, a glass substrate, a ceramic substrate, a quartz substrate, or a sapphire substrate may be used as the substrate 100. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate made of silicon, silicon carbide, or the like, a compound semiconductor substrate made of silicon germanium or the like, a silicon on insulator (SOI) substrate, or the like may be used as the substrate 100. Furthermore, any of these substrates further provided with a semiconductor element may be used as the substrate 100.

Alternatively, a flexible substrate may be used as the substrate 100, and the transistor 150 may be provided directly on the flexible substrate. Further alternatively, a separation layer may be provided between the substrate 100 and the transistor 150. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate 100 and transferred onto another substrate. In such a case, the transistor 150 can be transferred to a substrate having low heat resistance or a flexible substrate.

Examples of the base insulating film 102 include a silicon oxide film, a silicon oxynitride film, a silicon nitride film, a silicon nitride oxide film, a gallium oxide film, a hafnium oxide film, a yttrium oxide film, an aluminum oxide film, an aluminum oxynitride film, and the like.

In the case where the base insulating film 102 is formed using an oxide insulating film containing nitrogen and having a small number of defects, the gate insulating film 112 can be formed to have a single-layer structure or a stacked-layer structure using, for example, any of silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, a Ga—Zn-based metal oxide. Note that an oxide insulating film is preferably used for at least a region of the gate insulating film 112, which is positioned near the oxide semiconductor, in order to improve characteristics of the interface with the oxide semiconductor.

Furthermore, it is possible to prevent outward diffusion of oxygen from the oxide semiconductor and entry of hydrogen, water, and the like into the oxide semiconductor from the outside by providing an insulating film having a blocking effect against oxygen, hydrogen, water, and the like as the gate insulating film 112. As for the insulating film having a blocking effect against oxygen, hydrogen, water, and the like, an aluminum oxide film, an aluminum oxynitride film, a gallium oxide film, a gallium oxynitride film, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxide film, and a hafnium oxynitride film can be given as examples.

The gate insulating film 112 may be formed using a high-k material such as hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)), hafnium aluminate to which nitrogen is added (HfAl_(x)O_(y)), hafnium oxide, or yttrium oxide, so that gate leakage current of the transistor can be reduced.

The oxide semiconductor 104 is preferably formed using a metal oxide including at least In or Zn. The oxide semiconductor 104 is typically formed using an In—Ga oxide, an In—Zn oxide, an In—Mg oxide, a Zn—Mg oxide, an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, Mg, or Nd), or the like.

When an In-M-Zn oxide is used as the oxide semiconductor, the proportion of In and the proportion of M, with the exception of Zn and O, are preferably greater than or equal to 25 atomic % and less than 75 atomic %, respectively, further preferably greater than or equal to 34 atomic % and less than 66 atomic %, respectively.

Further, the energy gap of the oxide semiconductor is 2 eV or more, preferably 2.5 eV or more, more preferably 3 eV or more. With the use of an oxide semiconductor having such a wide energy gap, the off-state current of the transistor 150 can be reduced.

The thickness of the oxide semiconductor is greater than or equal to 3 nm and less than or equal to 200 nm, preferably greater than or equal to 3 nm and less than or equal to 100 nm, further preferably greater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor contains an In-M-Zn oxide (M represents Al, Ga, Y, Zr, La, Ce, Mg, or Nd), it is preferable that the atomic ratio of metal elements of a sputtering target used for forming a film of the In-M-Zn oxide satisfy In≧M and Zn≧M. As the atomic ratio of metal elements of such a sputtering target, In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, In:M:Zn=4:2:4.1, and the like are preferable. Note that the atomic ratios of metal elements in the formed oxide semiconductor vary from the above atomic ratio of metal elements of the sputtering target within a range of ±40% as an error.

Hydrogen contained in the oxide semiconductor reacts with oxygen bonded to a metal atom to be water, and also causes oxygen vacancies in a lattice from which oxygen is released (or a portion from which oxygen is released). Due to entry of hydrogen into the oxygen vacancies, an electron serving as a carrier is generated. Further, in some cases, bonding of part of hydrogen to oxygen bonded to a metal element causes generation of an electron serving as a carrier. Thus, a transistor including an oxide semiconductor which contains hydrogen is likely to be normally on.

It is thus preferable that hydrogen be reduced as much as possible as well as the oxygen vacancies in the oxide semiconductor. Specifically, the oxide semiconductor has a portion in which the concentration of hydrogen which is measured by secondary ion mass spectrometry (SIMS) is lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, more preferably lower than or equal to 1×10¹⁹ atoms/cm³, even more preferably lower than 5×10¹⁸ atoms/cm³, still more preferably lower than or equal to 1×10¹⁸ atoms/cm³, yet still more preferably lower than or equal to 5×10¹⁷ atoms/cm³, even further preferably lower than or equal to 1×10¹⁶ atoms/cm³. As a result, the transistor 150 has positive threshold voltage (normally-off characteristics).

When silicon or carbon that is one of elements belonging to Group 14 is contained in the oxide semiconductor, oxygen vacancies are increased in the oxide semiconductor, and the oxide semiconductor becomes an n-type. Thus, the oxide semiconductor has a portion in which the concentration of silicon or carbon (the concentration is measured by SIMS) is lower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

Furthermore, the oxide semiconductor has a portion in which the concentration of alkali metal or alkaline earth metal, which is measured by SIMS, is lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, which may increase the off-state current of the transistor. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal of the oxide semiconductor.

Furthermore, when nitrogen is contained in the oxide semiconductor, electrons serving as carriers are easily generated, so that carrier density is increased and the oxide semiconductor easily becomes an n-type semiconductor. Thus, a transistor including an oxide semiconductor that contains nitrogen is likely to be normally-on. For this reason, nitrogen in the oxide semiconductor is preferably reduced as much as possible. For example, the oxide semiconductor preferably includes a portion in which the concentration of nitrogen which is measured by SIMS is lower than or equal to 5×10¹⁸ atoms/cm³.

When impurities in the oxide semiconductor are reduced, the carrier density of the oxide semiconductor can be lowered. Thus, the oxide semiconductor preferably has a portion with a carrier density of 1×10¹⁷/cm³ or lower, further preferably 1×10¹⁵/cm³ or lower, still further preferably 1×10¹³/cm³ or lower, yet still further preferably 1×10¹¹/cm³ or lower.

The oxide semiconductor may have a non-single-crystal structure, for example. The non-single crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure described later, or an amorphous structure, for example. Among the non-single crystal structure, the amorphous structure has the highest density of defect levels, whereas CAAC-OS has the lowest density of defect levels.

Note that the oxide semiconductor may be a mixed film including two or more of the following: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a region of a CAAC-OS, and a region having a single-crystal structure. The mixed film includes, for example, two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases. Further, the mixed film has a stacked-layer structure of two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases.

The source electrode 108 a and the drain electrode 108 b can have a single-layer structure or a stacked-layer structure including any of metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten or an alloy containing any of these metals as its main component. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which an aluminum film is stacked over a tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a tungsten film, a three-layer structure in which a titanium film or a titanium nitride film, an aluminum film or a copper film, and a titanium film or a titanium nitride film are stacked in this order, a three-layer structure in which a molybdenum film or a molybdenum nitride film, an aluminum film or a copper film, and a molybdenum film or a molybdenum nitride film are stacked in this order, and the like can be given. Note that a transparent conductive material containing indium oxide, tin oxide, or zinc oxide may be used.

The gate electrode 114 can be formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloy containing any of these metal elements as a component; an alloy containing any of these metal elements in combination; or the like. Further, one or more metal elements selected from manganese and zirconium may be used. Further, the gate electrode 114 may have a single-layer structure or a stacked-layer structure of two or more layers. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which a titanium film is stacked over an aluminum film, a two-layer structure in which a titanium film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a titanium nitride film, a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film, a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order, and the like can be given. Alternatively, an alloy film or a nitride film in which aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium are combined may be used.

The gate electrode 114 can also be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, indium tin oxide containing silicon oxide, an indium oxide compound containing magnesium oxide, zinc oxide containing gallium oxide, zinc oxide containing aluminum oxide, zinc oxide containing magnesium oxide, or tin oxide containing fluorine. It is also possible to have a layered structure formed using the above light-transmitting conductive material and the above metal element.

Next, a method for manufacturing the transistor 150 illustrated in FIGS. 1A to 1C is described with reference to FIGS. 2A to 2C, FIGS. 3A to 3C, and FIGS. 4A and 4B. A cross-section in the channel length direction along dashed-dotted line A1-A2 in FIG. 1A is used in FIGS. 2A to 2C, FIGS. 3A to 3C, and FIGS. 4A and 4B to describe the method for manufacturing the transistor 150.

Films included in the transistor 150 (i.e., an insulating film, an oxide semiconductor, a metal oxide film, a conductive film, and the like) can be formed by any of a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, and a pulsed laser deposition (PLD) method. Alternatively, a coating method or a printing method can be used. Although the sputtering method and a plasma-enhanced chemical vapor deposition (PECVD) method are typical examples of the film formation method, a thermal CVD method may be used. As the thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method or an atomic layer deposition (ALD) method may be used, for example.

Deposition by a thermal CVD method may be performed in such a manner that a pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and a source gas is supplied to the chamber, and a reaction is caused in the vicinity of the substrate or over the substrate. Thus, no plasma is generated in the deposition; therefore, the thermal CVD method has an advantage that no defect due to plasma damage is caused.

Deposition by the ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). In such a case, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time or after the first source gas is introduced so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first single-atomic layer; then the second source gas is introduced to react with the first single-atomic layer; as a result, a second single-atomic layer is stacked over the first single-atomic layer, so that a thin film is formed.

The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute transistor.

First, the base insulating film 102 is formed over the substrate 100 (see FIG. 2A).

A glass substrate, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like can be used as the substrate 100. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like, a compound semiconductor substrate of silicon germanium or the like, a semiconductor-on-insulator (SOI) substrate, or the like may be used. Furthermore, any of these substrates provided with a semiconductor element may be used.

The base insulating film 102 can be formed by a plasma CVD method, a sputtering method, or the like using an oxide insulating film such as an aluminum oxide film, a magnesium oxide film, a silicon oxide film, a silicon oxynitride film, a gallium oxide film, a germanium oxide film, an yttrium oxide film, a zirconium oxide film, a lanthanum oxide film, a neodymium oxide film, a hafnium oxide film, a tantalum oxide film, or the like; a nitride insulating film such as a silicon nitride film, a silicon nitride oxide film, an aluminum nitride film, an aluminum nitride oxide film, or the like; or a mixed material of any of these. Alternatively, a stack including any of the above materials may be used, and an upper layer which is in contact with the oxide semiconductor is preferably formed using a material containing excess oxygen that might serve as a supply source of oxygen to the oxide semiconductor.

Oxygen may be added to the base insulating film 102 by an ion implantation method, an ion doping method, or the like. Adding oxygen enables the base insulating film 102 to supply oxygen more easily to the oxide semiconductor.

In the case where the base insulating film 102 is formed using a silicon oxide film or a silicon oxynitride film, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide can be given as examples.

In the case where a gallium oxide film is formed as the base insulating film 102, an MOCVD method can be used.

In the case where a hafnium oxide film is formed as the base insulating film 102 by a thermal CVD method such as an MOCVD method or an ALD method, two kinds of gases, i.e., ozone (03) as an oxidizer and a source material gas which is obtained by vaporizing a liquid containing a solvent and a hafnium precursor compound (a hafnium alkoxide or a hafnium amide such as tetrakis(dimethylamide)hafnium (TDMAH)), are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of another material liquid include tetrakis(ethylmethylamide)hafnium.

In the case where an aluminum oxide film is formed as the base insulating film 102 by a thermal CVD method such as an MOCVD method or an ALD method, two kinds of gases, i.e., H₂O as an oxidizer and a source material gas which is obtained by vaporizing a liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)), are used. Note that the chemical formula of trimethylaluminum is Al(CH₃)₃. Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris (2,2,6,6-tetramethyl-3,5-heptanedionate).

Furthermore, in the case where a silicon oxide film is formed as the base insulating film 102 by a thermal CVD method such as an MOCVD method or an ALD method, hexachlorodisilane is adsorbed on a deposition surface, chlorine contained in the adsorbate is removed, and radicals of an oxidizing gas (e.g., oxygen or dinitrogen monoxide) are supplied to react with the adsorbate.

Here, a silicon oxynitride film is formed as the base insulating film 102 by a PECVD method.

In the case where a surface of the substrate 100 is made of an insulator and there is no influence of impurity diffusion into the oxide semiconductor to be formed later, the base insulating film 102 is not necessarily provided.

Next, the oxide semiconductor is formed over the base insulating film 102 by a sputtering method, a CVD method, an MBE method, an ALD method, a PLD method, or the like.

Alternatively, the oxide semiconductor may be formed by stacking layers having different compositions. In the case where the oxide semiconductor is formed by stacking layers, in order to form a continuous conjunction at an interface between layers, the layers are preferably stacked successively without exposure to the air with the use of a multi-chamber deposition apparatus (e.g., a sputtering apparatus) including a load lock chamber. It is preferable that each chamber of the sputtering apparatus be able to be evacuated to a high vacuum (approximately 5×10⁻⁷ Pa to 1×10⁻⁴ Pa) by an adsorption vacuum evacuation pump such as a cryopump and that the chamber be able to heat a substrate over which a film is to be deposited to 100° C. or higher, preferably 200° C. or higher, so that water and the like acting as impurities of an oxide semiconductor are removed as much as possible. Alternatively, a combination of a turbo molecular pump and a cold trap is preferably used to prevent back-flow of a gas containing a carbon component, moisture, or the like from an exhaust system into the chamber.

Not only high vacuum evacuation of the chamber but also high purity of a sputtering gas is necessary to obtain a highly purified intrinsic oxide semiconductor. When a highly purified gas having a dew point of −40° C. or lower, preferably −80° C. or lower, more preferably −100° C. or lower is used as an oxygen gas or an argon gas used as a sputtering gas, moisture or the like can be prevented from entering an oxide semiconductor as much as possible.

A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources, and thus can have a low carrier density. Thus, a transistor including the oxide semiconductor rarely has a negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier traps. Accordingly, the transistor including the oxide semiconductor has little variation in electrical characteristics and high reliability. An electric charge trapped by the carrier traps in the oxide semiconductor takes a long time to be released. The trapped electric charge may behave like a fixed electric charge. Thus, the transistor which includes the oxide semiconductor having a high impurity concentration and a high density of defect states might have unstable electrical characteristics.

The oxide which can be used as the oxide semiconductor preferably contains at least indium (In) or zinc (Zn). Alternatively, both In and Zn are preferably contained. In order to reduce fluctuations in electrical characteristics of the transistors including the oxide semiconductor, the oxide semiconductor preferably contains a stabilizer in addition to In and Zn.

As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al), zirconium (Zr), and the like can be given. As another stabilizer, lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) can be given.

As the oxide semiconductor, for example, any of the following can be used: indium oxide, tin oxide, zinc oxide, an In—Zn oxide, a Sn—Zn oxide, an Al—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, an In—Ga oxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, an In—Sn—Zn oxide, a Sn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Hf—Zn oxide, an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Zn oxide, an In—Nd—Zn oxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, an In—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide, an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Zn oxide, an In—Sn—Ga—Zn oxide, an In—Hf—Ga—Zn oxide, an In—Al—Ga—Zn oxide, an In—Sn—Al—Zn oxide, an In—Sn—Hf—Zn oxide, or an In—Hf—Al—Zn oxide.

Note that here, for example, an “In—Ga—Zn oxide” means an oxide containing In, Ga, and Zn as its main components. The In—Ga—Zn oxide may contain a metal element other than the In, Ga, and Zn. Further, in this specification, a film formed using an In—Ga—Zn oxide is also referred to as an IGZO film.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 is satisfied, and m is not an integer) may be used. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co. Alternatively, a material represented by In₂SnO₅(ZnO)_(n) (n>0, n is an integer) may be used.

Note that the oxide semiconductor is preferably formed by a sputtering method. As a sputtering method, an RF sputtering method, a DC sputtering method, an AC sputtering method, or the like can be used. In particular, a DC sputtering method is preferably used because dust generated in the deposition can be reduced and the film thickness can be uniform.

As a sputtering gas, a rare gas (typically argon), an oxygen, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen to a rare gas is preferably increased.

Furthermore, a target may be appropriately selected in accordance with the composition of the oxide semiconductor to be formed.

For example, in the case where the oxide semiconductor is formed by a sputtering method at a substrate temperature higher than or equal to 150° C. and lower than or equal to 750° C., preferably higher than or equal to 150° C. and lower than or equal to 450° C., further preferably higher than or equal to 200° C. and lower than or equal to 350° C., the oxide semiconductor can be a CAAC-OS film.

For the deposition of the CAAC-OS film, the following conditions are preferably used.

By suppressing entry of impurities into the CAAC-OS film during the deposition, the crystal state can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen) which exist in the deposition chamber may be reduced. Furthermore, the concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is −80° C. or lower, preferably −100° C. or lower is used.

Furthermore, preferably, the proportion of oxygen in the sputtering gas is increased and the power is optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the sputtering gas is 30 vol % or higher, preferably 100 vol %.

In addition, by forming the oxide semiconductor while it is heated or performing heat treatment after the formation of the oxide semiconductor, the oxide semiconductor can have a portion in which the hydrogen concentration is lower than or equal to 2×10²⁰ atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, more preferably lower than or equal to 1×10¹⁹ atoms/cm³, even more preferably lower than 5×10¹⁸ atoms/cm³, still more preferably lower than or equal to 1×10¹⁸ atoms/cm³, yet still more preferably lower than or equal to 5×10¹⁷ atoms/cm³, even further preferably lower than or equal to 1×10¹⁶ atoms/cm³.

After the oxide semiconductor is formed, dehydrogenation or dehydration may be performed by heat treatment. The temperature of the heat treatment is typically higher than or equal to 150° C. and lower than the strain point of the substrate, preferably higher than or equal to 250° C. and lower than or equal to 450° C., more preferably higher than or equal to 300° C. and lower than or equal to 450° C.

The heat treatment is performed under an inert gas atmosphere containing nitrogen or a rare gas such as helium, neon, argon, xenon, or krypton. Further, the heat treatment may be performed under an inert gas atmosphere first, and then under an oxygen atmosphere. It is preferable that the above inert gas atmosphere and the above oxygen atmosphere do not contain hydrogen, water, and the like. The treatment time is 3 minutes to 24 hours.

An electric furnace, a rapid thermal annealing (RTA) apparatus, or the like can be used for the heat treatment. With the use of an RTA apparatus, the heat treatment can be performed at a temperature of higher than or equal to the strain point of the substrate if the heating time is short. Therefore, the heat treatment time can be shortened.

When the heat treatment is performed at temperatures higher than 350° C. and lower than or equal to 650° C., preferably higher than or equal to 450° C. and lower than or equal to 600° C., it is possible to obtain an oxide semiconductor whose proportion of CAAC is greater than or equal to 60%, preferably greater than or equal to 80%, further preferably greater than or equal to 90%, still further preferably greater than or equal to 95%. Furthermore, it is possible to obtain an oxide semiconductor having a low content of hydrogen, water, and the like. That is, an oxide semiconductor with a low impurity concentration and a low density of defect states can be formed. Note that even when the oxide semiconductor is a CAAC-OS film, a diffraction pattern similar to that of an nc-OS film or the like is partly observed in some cases. The proportion of a region where a diffraction pattern of a CAAC-OS film is observed in a predetermined area is defined as the proportion of CAAC.

For example, when an oxide semiconductor, e.g., an InGaZnOx (X>0) film is formed using a deposition apparatus employing an ALD method, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced more than once to form an InO₂ layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at the same time to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas are introduced at the same time to form a ZnO layer. Note that the order of these layers is not limited to this example. A mixed compound layer such as an InGaO₂ layer, an InZnO₂ layer, a GaInO layer, a ZnInO layer, or a GaZnO layer may be formed by mixing of these gases. Note that although an H₂O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an O₃ gas, which does not contain H. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Alternatively, a Zn(CH₃)₂ gas may be used.

Next, an oxide semiconductor is formed by, for example, a sputtering method, a mask is formed over the oxide semiconductor, and then part of the oxide semiconductor is selectively etched. Next, the mask is removed, whereby the oxide semiconductor 104 is formed (see FIG. 2B).

At this time, as shown in the drawing, the base insulating film 102 can be slightly overetched.

For processing into island shapes such as the oxide semiconductor 104, first, a film to be a hard mask (e.g., a tungsten film) and a resist mask are provided over the oxide semiconductor 104, and the film to be a hard mask is etched into a hard mask. Then, the resist mask is removed, and with use of the hard mask as a mask, the oxide semiconductor 104 is etched. Then, the hard mask is removed. At this step, the hard mask is gradually reduced as the etching progresses; as a result, the edges of the hard mask is rounded to have a curved surface. Accordingly, end portions of the oxide semiconductor 104 are also rounded to have curved surfaces. With this structure, the coverage with the oxide semiconductor 110, the gate insulating film 112, the gate electrode 114, and the insulating film 116, which are to be formed over the oxide semiconductor 104 can be improved; thus, a shape defect such as disconnection can be inhibited.

Next, the insulating film 103 is formed over the oxide semiconductor 104 (see FIG. 2C). The insulating film 103 can be formed using a material and a method similar to those of the base insulating film 102. In particular, the insulating film 103 preferably has a function of releasing oxygen by heating. Accordingly, oxygen can be supplied from the insulating film 103 to the oxide semiconductor 104, whereby electrical characteristics of the transistor including the oxide semiconductor can be improved.

Next, as illustrated in FIG. 3A, a surface of the insulating film 103 is polished by chemical mechanical polishing (CMP) treatment or the like, whereby an insulating film 105 is formed. In FIG. 3A, the oxide semiconductor 104 is not exposed, but CMP or the like may be performed until the oxide semiconductor 104 is exposed.

By polishing the insulating film in such a manner, the thickness of the insulating film 105 over the base insulating film 102 can be made different from that of the insulating film 105 over the oxide semiconductor 104. Alternatively, the insulating film 103 illustrated in FIG. 2C may be formed with a coating film using a siloxane or the like. Thereby, this case is preferable because polishing by the above described CMP or the like is not needed.

Next, the insulating film 105 is etched with use of a dry etching apparatus and the like, whereby the insulating film 106 is formed (see FIG. 3B). Part of the oxide semiconductor 104 is exposed by the etching, so that the first portion 104 a which is not exposed from the insulating film 106 and the second portion 104 b which is exposed from the insulating film 106 are formed.

The insulating film 106 in contact with the oxide semiconductor 104 is formed as described above, so that oxygen can be supplied from the insulating film 106 to the side surfaces of the first portion 104 a of the oxide semiconductor 104.

Next, the source electrode 108 a and the drain electrode 108 b are formed so as to be in contact with the oxide semiconductor 104 (see FIG. 3C).

Next, the oxide semiconductor 107 is formed over the oxide semiconductor 104, the source electrode 108 a, and the drain electrode 108 b. The insulating film 109 is formed over the oxide semiconductor 107 (see FIG. 4A).

Note that after the oxide semiconductor 107 is formed, heat treatment may be performed. The heat treatment can remove impurities such as hydrogen and water from the oxide semiconductor 107. In addition, impurities such as hydrogen and water can be further removed from the oxide semiconductor 104.

Next, the gate electrode 114 is formed so as to overlap with the oxide semiconductor 104 with the insulating film 109 provided therebetween (see FIG. 4B).

In FIG. 4B, after the processing of the gate electrode 114, the insulating film 109 and the oxide semiconductor 107 are etched using the gate electrode 114 as a mask, whereby the oxide semiconductor 110 and the gate insulating film 112 are formed. However, the insulating film 109 and the oxide semiconductor 107 are not necessarily etched after the gate electrode 114 is formed.

Next, the insulating film 116 is formed over the gate insulating film 112 and the gate electrode 114 (see FIG. 4B).

The insulating film 116 is preferably formed by an ALD method. Owing to good coverage, a film formed by an ALD method can favorably cover a large step portion (such as a step formed by the gate electrode 114 and the gate insulating film 112) and can stabilize the characteristics of the transistor 150.

Through the above process, the transistor 150 can be manufactured.

Modification Example 1

Alternatively, a transistor 152 obtained by adding a back gate electrode 120 to the transistor 150 may be provided. The cross-sectional view of the transistor 152 in a channel length direction is illustrated in FIG. 5A. By providing the back gate electrode 120 in this manner, characteristics of the transistor (especially a threshold voltage) can be controlled. The back gate electrode 120 can be formed using a material and a method that are similar to those of the gate electrode 114 of the transistor 150. The back gate electrode 120 may be electrically connected to the gate electrode.

Modification Example 2

Alternatively, as illustrated in FIG. 5B, a transistor 154 can have a self-aligned structure in which low-resistance regions 142 are formed by reducing the resistance of the oxide semiconductor 140 in offset regions where the gate electrode 130 does not overlap with the source electrode 133 a and the drain electrode 133 b.

The low-resistance regions 142 exhibiting n-type conductivity can be formed by adding an impurity to the oxide semiconductor 140 using the gate electrode 130 as a mask. Examples of the method for adding the impurity include an ion implantation method and an ion doping method.

An impurity such as hydrogen, helium, neon, argon, krypton, xenon, boron, nitrogen, phosphorus, or arsenic increases the conductivities of the oxide semiconductor 140.

The impurity may be added to the whole or part of the first portion 140 a of the oxide semiconductor 140, or the impurity may be added to whole or part of the second portion 140 b of the oxide semiconductor 140. Alternatively, the impurity may be added to the whole or part of the first portion 140 a and the second portion 140 b.

Modification Example 3

The transistor 150 described in this embodiment includes two oxide semiconductor layers (the oxide semiconductor 104 and the oxide semiconductor 110), but may include a single oxide semiconductor layer or three, four, or more oxide semiconductor layers. For example, as in a transistor 160 in FIG. 6A, the oxide semiconductor 110 may be removed from the transistor 150 (i.e., the oxide semiconductor is a single layer). As illustrated in FIG. 6A, a gate insulating film 118 may be formed in contact with the oxide semiconductor 104.

In addition, as in a transistor 162 illustrated in FIG. 6B, a first portion 204 a of an oxide semiconductor 204 may include two oxide semiconductor layers (an oxide semiconductor 211 and an oxide semiconductor 213). Alternatively, the first portion 204 a may include two or more oxide semiconductor layers. Alternatively, a second portion 204 b may include two or more oxide semiconductor layers. Alternatively, each of the first portion 204 a and the second portion 204 b may include two or more oxide semiconductor layers.

Furthermore, the same oxide semiconductor may be used for at least parts of the first portion and the second portion of the oxide semiconductor. For example, as in a transistor 164 illustrated in FIG. 6C, an oxide semiconductor 217 may be used for a first portion 304 a and part of a second portion 304 b in an oxide semiconductor 304. In addition, an oxide semiconductor 219 may be used for part of the first portion 304 a and the second portion 304 b.

The structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments and examples.

Embodiment 2

In this embodiment, one embodiment that can be applied to the oxide semiconductor in any of the transistors included in the semiconductor device described in the above embodiment is described.

Oxide semiconductors are classified roughly into a single-crystal oxide semiconductor and a non-single-crystal oxide semiconductor. The non-single-crystal oxide semiconductor includes any of a c-axis aligned crystalline oxide semiconductor (CAAC-OS), a polycrystalline oxide semiconductor, a microcrystalline oxide semiconductor, an amorphous oxide semiconductor, and the like.

First, a CAAC-OS film is described.

A CAAC-OS film is an oxide semiconductor having a plurality of c-axis aligned crystal parts.

With a transmission electron microscope (TEM), a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of the CAAC-OS film is observed. Consequently, a plurality of crystal parts can be observed. However, in the high-resolution TEM image, a boundary between crystal parts, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS film, a reduction in electron mobility due to the grain boundary is less likely to occur.

According to the high-resolution cross-sectional TEM image of the CAAC-OS film observed in a direction substantially parallel to the sample surface, metal atoms are arranged in a layered manner in the crystal parts. Each metal atom layer has a morphology reflecting unevenness of a surface over which the CAAC-OS film is formed (hereinafter, a surface over which the CAAC-OS film is formed is referred to as a formation surface) or a top surface of the CAAC-OS film, and is arranged parallel to the formation surface or the top surface of the CAAC-OS film.

According to the high-resolution plan TEM image of the CAAC-OS film observed in a direction substantially perpendicular to the sample surface, metal atoms are arranged in a triangular or hexagonal configuration in the crystal parts. However, there is no regularity of arrangement of metal atoms between different crystal parts.

A CAAC-OS film is subjected to structural analysis with an X-ray diffraction (XRD) apparatus. For example, when the CAAC-OS film including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears frequently when the diffraction angle (2θ) is around 31°. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS film have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS film.

Note that when the CAAC-OS film with an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak of 28 may also be observed at around 36°, in addition to the peak of 2θ at around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS film. It is preferable that in the CAAC-OS film, a peak of 2θ appears at around 31° and a peak of 2θ not appear at around 36°.

The CAAC-OS film is an oxide semiconductor with a low impurity concentration. The impurity means here an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. In particular, an element that has higher bonding strength to oxygen than a metal element included in the oxide semiconductor, such as silicon, disturbs the atomic arrangement of the oxide semiconductor by depriving the oxide semiconductor of oxygen and causes a decrease in crystallinity. A heavy metal such as iron or nickel, argon, carbon dioxide, or the like has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity. Additionally, the impurity contained in the oxide semiconductor might serve as a carrier trap or a carrier generation source.

In addition, the CAAC-OS film is an oxide semiconductor having a low density of defect states. For example, oxygen vacancies in the oxide semiconductor serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.

The state in which impurity concentration is low and density of defect states is low (the number of oxygen vacancies is small) is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources, and thus has a low carrier density in some cases. Thus, a transistor including the oxide semiconductor rarely has a negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier traps. Accordingly, the transistor including the oxide semiconductor has little variation in electrical characteristics and high reliability. An electric charge trapped by the carrier traps in the oxide semiconductor takes a long time to be released. The trapped electric charge may behave like a fixed electric charge. Thus, the transistor which includes the oxide semiconductor having a high impurity concentration and a high density of defect states might have unstable electrical characteristics.

With the use of the CAAC-OS film in a transistor, variation in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light is small.

Next, a microcrystalline oxide semiconductor is described.

A microcrystalline oxide semiconductor has a region in which a crystal part is observed and a region in which a crystal part is not observed clearly in a high-resolution TEM image. In most cases, the size of a crystal part included in the microcrystalline oxide semiconductor is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. An oxide semiconductor including a nanocrystal (nc) that is a microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as a nanocrystalline oxide semiconductor (nc-OS) film. In a high resolution TEM image of the nc-OS film, a grain boundary cannot be found clearly in the nc-OS film sometimes for example.

In the nc-OS film, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic order. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS film. Thus, the orientation of the whole film is not observed. Accordingly, the nc-OS film cannot be distinguished from an amorphous oxide semiconductor, depending on an analysis method. For example, when the nc-OS film is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than that of a crystal part, a peak which shows a crystal plane does not appear. Further, a halo pattern is shown in an electron diffraction pattern (also referred to as a selected-area electron diffraction pattern) of the nc-OS film obtained by using an electron beam having a probe diameter (e.g., larger than or equal to 50 nm) larger than the diameter of a crystal part. Meanwhile, spots are shown in a nanobeam electron diffraction pattern of the nc-OS film obtained by using an electron beam having a probe diameter close to, or smaller than the diameter of a crystal part. Further, in a nanobeam electron diffraction pattern of the nc-OS film, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS film, a plurality of spots is shown in a ring-like region in some cases.

The nc-OS film is an oxide semiconductor that has high regularity as compared with an amorphous oxide semiconductor. Thus, the nc-OS film has a lower density of defect states than the amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different crystal parts in the nc-OS film. Therefore, the nc-OS film has a higher density of defect states than the CAAC-OS film.

Next, an amorphous oxide semiconductor is described.

The amorphous oxide semiconductor is such an oxide semiconductor having disordered atomic arrangement and no crystal part. For example, the amorphous oxide semiconductor does not have a specific state as in quartz.

In a high-resolution TEM image of the amorphous oxide semiconductor, crystal parts cannot be found.

When the amorphous oxide semiconductor is subjected to structural analysis by an out-of-plane method with an XRD apparatus, a peak which shows a crystal plane does not appear. A halo pattern is observed when the amorphous oxide semiconductor is subjected to electron diffraction. Furthermore, a spot is not observed and a halo pattern appears when the amorphous oxide semiconductor is subjected to nanobeam electron diffraction.

Note that an oxide semiconductor may have a structure having physical properties intermediate between the nc-OS film and the amorphous oxide semiconductor. The oxide semiconductor having such a structure is specifically referred to as an amorphous-like oxide semiconductor (a-like OS) film.

In a high-resolution TEM image of the amorphous-like OS film, a void may be seen. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed. In the amorphous-like OS film, crystallization by a slight amount of electron beam used for TEM observation occurs and growth of the crystal part is found sometimes. In contrast, crystallization by a slight amount of electron beam used for TEM observation is less observed in the nc-OS film having good quality.

Note that the crystal part size in the amorphous-like OS film and the nc-OS film can be measured using high-resolution TEM images. For example, an InGaZnO₄ crystal has a layered structure in which two Ga—Zn—O layers are included between In—O layers. A unit cell of the InGaZnO₄ crystal has a structure in which nine layers of three In—O layers and six Ga—Zn—O layers are layered in the c-axis direction. Accordingly, the spacing between these adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to 0.29 nm from crystal structure analysis. Thus, focusing on lattice fringes in the high-resolution TEM image, each of lattice fringes in which the lattice spacing therebetween is greater than or equal to 0.28 nm and less than or equal to 0.30 nm corresponds to the a-b plane of the InGaZnO₄ crystal.

Note that an oxide semiconductor may be a stacked-layer film including two or more films of an amorphous oxide semiconductor, an amorphous-like OS film, a microcrystalline oxide semiconductor, and a CAAC-OS film, for example.

Note that the structures, methods, and the like described in this embodiment can be used as appropriate in combination with any of the structures, methods, and the like described in the other embodiments and examples.

Embodiment 3

In this embodiment, an example of a circuit including the transistor of one embodiment of the present invention is described with reference to drawings.

<Cross-Sectional Structure>

FIG. 7A is a cross-sectional view of a semiconductor device of one embodiment of the present invention. The semiconductor device illustrated in FIG. 7A includes a transistor 2200 containing a first semiconductor material in a lower portion and a transistor 2100 containing a second semiconductor material in an upper portion. As the transistor 2100, any of the transistors described in the above embodiment can be used, and in FIG. 7A, an example in which the transistor 150 is used as the transistor 2100 is shown. A cross-sectional view of the transistors in a channel length direction is on the left side of a dashed-dotted line, and a cross-sectional view of the transistors in a channel width direction is on the right side of the dashed-dotted line.

Note that the transistor 2100 may be provided with a back gate.

Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material can be a semiconductor material other than an oxide semiconductor (examples of such a semiconductor material include silicon (including strained silicon), germanium, silicon germanium, silicon carbide, gallium arsenide, aluminum gallium arsenide, indium phosphide, gallium nitride, and an organic semiconductor), and the second semiconductor material can be an oxide semiconductor. A transistor using a material other than an oxide semiconductor, such as single crystal silicon, can operate at high speed easily. On the other hand, a transistor including an oxide semiconductor has a low off-state current.

The transistor 2200 may be either an n-channel transistor or a p-channel transistor, and an appropriate transistor may be used in accordance with a circuit. Furthermore, the specific structure of the semiconductor device, such as the material or the structure used for the semiconductor device, is not necessarily limited to those described here except for the use of the transistor of one embodiment of the present invention which uses an oxide semiconductor.

FIG. 7A illustrates a structure in which the transistor 2100 is provided over the transistor 2200 with an insulating film 2201 and an insulating film 2207 provided therebetween. A plurality of wirings 2202 are provided between the transistor 2200 and the transistor 2100. Furthermore, wirings and electrodes provided over and under the insulating films are electrically connected to each other through a plurality of plugs 2203 embedded in the insulating films. An insulating film 2204 covering the transistor 2100, a wiring 2205 over the insulating film 2204, and a wiring 2206 formed by processing a conductive film that is also used for a pair of electrodes of the transistor 2100 are provided.

The stack of the two kinds of transistors reduces the area occupied by the circuit, allowing a plurality of circuits to be highly integrated.

Here, in the case where a silicon-based semiconductor material is used for the transistor 2200 provided in a lower portion, hydrogen in an insulating film provided in the vicinity of the semiconductor film of the transistor 2200 terminates dangling bonds of silicon; accordingly, the reliability of the transistor 2200 can be improved. Meanwhile, in the case where an oxide semiconductor is used for the transistor 2100 provided in an upper portion, hydrogen in an insulating film provided in the vicinity of the semiconductor film of the transistor 2100 becomes a factor of generating carriers in the oxide semiconductor; thus, the reliability of the transistor 2100 might be decreased. Therefore, in the case where the transistor 2100 using an oxide semiconductor is provided over the transistor 2200 using a silicon-based semiconductor material, it is particularly effective that the insulating film 2207 having a function of preventing diffusion of hydrogen is provided between the transistors 2100 and 2200. The insulating film 2207 makes hydrogen remain in the lower portion, thereby improving the reliability of the transistor 2200. In addition, since the insulating film 2207 suppresses diffusion of hydrogen from the lower portion to the upper portion, the reliability of the transistor 2100 also can be improved.

The insulating film 2207 can be, for example, formed using aluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, hafnium oxynitride, or yttria-stabilized zirconia (YSZ).

Furthermore, a blocking film 2208 (corresponding to the insulating film 116 in the transistor 150) having a function of preventing diffusion of hydrogen is preferably formed over the transistor 2100 to cover the transistor 2100 including an oxide semiconductor. For the blocking film 2208, a material that is similar to that of the insulating film 2207 can be used, and in particular, an aluminum oxide film is preferably used. The aluminum oxide film has a high shielding (blocking) effect of preventing penetration of both oxygen and impurities such as hydrogen and moisture. Thus, by using the aluminum oxide film as the blocking film 2208 covering the transistor 2100, release of oxygen from the oxide semiconductor included in the transistor 2100 can be prevented and entry of water and hydrogen into the oxide semiconductor can be prevented.

Note that the transistor 2200 can be a transistor of various types without being limited to a planar type transistor. For example, the transistor 2200 can be a fin-type transistor, a tri-gate transistor, or the like. An example of a cross-sectional view in this case is shown in FIG. 7D. An insulating film 2212 is provided over a semiconductor substrate 2211. The semiconductor substrate 2211 includes a projecting portion with a thin tip (also referred to a fin). Note that an insulating film may be provided over the projecting portion. The insulating film functions as a mask for preventing the semiconductor substrate 2211 from being etched when the projecting portion is formed. Alternatively, the projecting portion may not have the thin tip; a projecting portion with a cuboid-like projecting portion and a projecting portion with a thick tip are permitted, for example. A gate insulating film 2214 is provided over the projecting portion of the semiconductor substrate 2211, and a gate electrode 2213 is provided over the gate insulating film 2214. Although the gate electrode 2213 has a two-layer structure in this embodiment, the present invention is not limited to this example, and the gate electrode 2213 may have a single-layer structure or a multilayer structure including three or more layers. Source and drain regions 2215 are formed in the semiconductor substrate 2211. Note that here is shown an example in which the semiconductor substrate 2211 includes the projecting portion; however, a semiconductor device of one embodiment of the present invention is not limited thereto. For example, a semiconductor region having a projecting portion may be formed by processing an SOI substrate.

<Example of Circuit Configuration>

In the above structure, electrodes of the transistor 2100 and the transistor 2200 can be connected in a variety of ways; thus, a variety of circuits can be formed. Examples of circuit configurations which can be achieved by using a semiconductor device of one embodiment of the present invention are shown below.

<CMOS Circuit>

A circuit diagram in FIG. 7B shows a configuration of a so-called CMOS circuit in which the p-channel transistor 2200 and the n-channel transistor 2100 are connected to each other in series and in which gates of them are connected to each other.

<Analog Switch>

A circuit diagram in FIG. 7C shows a configuration in which sources of the transistors 2100 and 2200 are connected to each other and drains of the transistors 2100 and 2200 are connected to each other. With such a configuration, the transistors can function as a so-called analog switch.

<Example of Memory Device>

An example of a semiconductor device (memory device) which includes the transistor of one embodiment of the present invention, which can retain stored data even when not powered, and which has an unlimited number of write cycles is shown in FIGS. 8A to 8C.

The semiconductor device illustrated in FIG. 8A includes a transistor 3200 including a first semiconductor material, a transistor 3300 including a second semiconductor material, and a capacitor 3400. As the transistor 3300, the transistor described in the above embodiments can be used.

FIG. 8B is a cross-sectional view of the semiconductor device illustrated in FIG. 8A. The semiconductor device in the cross-sectional view has a structure in which the transistor 3300 is provided with a back gate.

The transistor 3300 is a transistor in which a channel is formed in a semiconductor film including an oxide semiconductor. Since the off-state current of the transistor 3300 is low, stored data can be retained for a long period owing to such a transistor. In other words, power consumption can be sufficiently reduced because a semiconductor device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided.

In FIG. 8A, a first wiring 3001 is electrically connected to a source electrode of the transistor 3200. A second wiring 3002 is electrically connected to a drain electrode of the transistor 3200. A third wiring 3003 is electrically connected to one of the source electrode and the drain electrode of the transistor 3300. A fourth wiring 3004 is electrically connected to the gate electrode of the transistor 3300. A gate electrode of the transistor 3200 is electrically connected to the other of the source electrode and the drain electrode of the transistor 3300 and one electrode of the capacitor 3400. A fifth wiring 3005 is electrically connected to the other electrode of the capacitor 3400.

The semiconductor device in FIG. 8A utilizes a feature that the potential of the gate electrode of the transistor 3200 can be retained, and thus enables writing, retaining, and reading of data as follows.

Writing and holding of data will be described. First, the potential of the fourth wiring 3004 is set to a potential at which the transistor 3300 is turned on, so that the transistor 3300 is turned on. Accordingly, the potential of the third wiring 3003 is supplied to the gate electrode of the transistor 3200 and the capacitor 3400. That is, a predetermined charge is supplied to the gate of the transistor 3200 (writing). Here, charge for supplying either of two different potential levels (hereinafter referred to as low-level charge and high-level charge) is given. After that, the potential of the fourth wiring 3004 is set to a potential at which the transistor 3300 is turned off, so that the transistor 3300 is turned off Thus, the charge supplied to the gate of the transistor 3200 is retained (retaining).

Since the off-state current of the transistor 3300 is extremely low, the charge of the gate of the transistor 3200 is retained for a long time.

Next, reading of data will be described. An appropriate potential (a reading potential) is supplied to the fifth wiring 3005 while a predetermined potential (a constant potential) is supplied to the first wiring 3001, whereby the potential of the second wiring 3002 varies depending on the amount of charge retained in the gate of the transistor 3200. This is because in general, when the transistor 3200 is an n-channel transistor, an apparent threshold voltage V_(th) _(—) _(H) in the case where a high-level charge is given to the gate electrode of the transistor 3200 is lower than an apparent threshold voltage V_(th) _(—) _(L) in the case where a low-level charge is given to the gate electrode of the transistor 3200. Here, an apparent threshold voltage refers to the potential of the fifth wiring 3005 which is needed to turn on the transistor 3200. Thus, the potential of the fifth wiring 3005 is set to a potential V₀ which is between V_(th) _(—) _(H) and V_(th) _(—) _(L), whereby charge supplied to the gate of the transistor 3200 can be determined. For example, in the case where the high-level charge is supplied in writing, when the potential of the fifth wiring 3005 is V₀ (>V_(th) _(—) _(H)), the transistor 3200 is turned on. In the case where the low-level charge is supplied in writing, even when the potential of the fifth wiring 3005 is V₀ (<V_(th) _(—) _(L)), the transistor 3200 remains off. Thus, the data retained in the gate electrode of the transistor 3200 can be read by determining the potential of the second wiring 3002.

Note that in the case where memory cells are arrayed to be used, only data of desired memory cells needs to be read. The fifth wiring 3005 in the case where data is not read may be supplied with a potential at which the transistor 3200 is turned off regardless of the state of the gate, that is, a potential lower than V_(th) _(—) _(H). Alternatively, the fifth wiring 3005 may be supplied with a potential at which the transistor 3200 is turned on regardless of the state of the gate, that is, a potential higher than V_(th) _(—) _(L).

A semiconductor device illustrated in FIG. 8C is different from the semiconductor device illustrated in FIG. 8A in that the transistor 3200 is not provided. Also in this case, writing and holding of data can be performed in a manner similar to the above.

Next, operation of data reading will be described. When the transistor 3300 is turned on, the third wiring 3003 which is in a floating state and the capacitor 3400 are electrically connected to each other, and the charge is redistributed between the third wiring 3003 and the capacitor 3400. As a result, the potential of the third wiring 3003 is changed. The amount of change in potential of the third wiring 3003 varies depending on the potential of the one electrode of the capacitor 3400 (or the charge accumulated in the capacitor 3400).

For example, the potential of the third wiring 3003 after the charge redistribution is (C_(B)×V_(B0)+C×V) (C_(B)+C), where V is the potential of the one electrode of the capacitor 3400, C is the capacitance of the capacitor 3400, C_(B) is the capacitance component of the third wiring 3003, and V_(B0) is the potential of the third wiring 3003 before the charge redistribution. Thus, it can be found that, assuming that the memory cell is in either of two states in which the potential of the one electrode of the capacitor 3400 is V₁ and V₀ (V₁>V₀), the potential of the third wiring 3003 in the case of retaining the potential V₁(=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of the third wiring 3003 in the case of retaining the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the third wiring 3003 with a predetermined potential, data can be read.

In this case, a transistor including the first semiconductor material may be used for a driver circuit for driving a memory cell, and a transistor including the second semiconductor material may be stacked over the driver circuit as the transistor 3300.

When a transistor having a channel formation region formed using an oxide semiconductor and having extremely small off-state current is applied to the semiconductor device in this embodiment, the semiconductor device can store data for an extremely long period. In other words, power consumption can be sufficiently reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional nonvolatile memory, it is not necessary to inject and extract electrons into and from a floating gate, and thus a problem such as deterioration of a gate insulating film does not arise at all. In other words, the semiconductor device according to one embodiment of the present invention does not have a limit on the number of times of writing which is a problem in a conventional nonvolatile memory, and reliability thereof is drastically improved. Furthermore, data is written depending on the on state and the off state of the transistor, whereby high-speed operation can be easily achieved.

This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 4

In this embodiment, an RFID tag that includes the transistor described in the above embodiments or the memory device described in the above embodiment is described with reference to FIG. 9.

The RFID tag in this embodiment includes a memory circuit inside, stores information which is necessary for the memory circuit, and transmits and receives information to/from the outside by using contactless means, for example, wireless communication. With these characteristics, the RFID tag can be used for an individual authentication system in which an object or the like is recognized by reading the individual information, for example. In order that the RFID tag is used for such application, extremely high reliability is needed.

A configuration of the RFID tag will be described with reference to FIG. 9. FIG. 9 is a block diagram illustrating a configuration example of the RFID tag.

As shown in FIG. 9, an RFID tag 800 includes an antenna 804 which receives a radio signal 803 that is transmitted from an antenna 802 connected to a communication device 801 (also referred to as an interrogator, a reader/writer, or the like). The RFID tag 800 includes a rectifier circuit 805, a constant voltage circuit 806, a demodulation circuit 807, a modulation circuit 808, a logic circuit 809, a memory circuit 810, and a ROM 811. A transistor having a rectifying function included in the demodulation circuit 807 may be formed using a material which enables a reverse current to be low enough, for example, an oxide semiconductor. This can suppress the phenomenon of a rectifying function becoming weaker due to generation of a reverse current and prevent saturation of the output from the demodulation circuit. In other words, the input to the demodulation circuit and the output from the demodulation circuit can have a relation closer to a linear relation. Note that data transmission methods are roughly classified into the following three methods: an electromagnetic coupling method by which a pair of coils is provided so as to be faced with each other and communicates with each other by mutual induction, an electromagnetic induction method by which communication is performed using an induction field, and an electric wave method by which communication is performed using an electric wave. Any of these methods can be used in the RFID tag 800 described in this embodiment.

Next, a structure of each circuit will be described. The antenna 804 exchanges the radio signal 803 with the antenna 802 which is connected to the communication device 801. The rectifier circuit 805 generates an input potential by rectification, for example, half-wave voltage doubler rectification of an input alternating signal generated by reception of a radio signal at the antenna 804 and smoothing of the rectified signal with a capacitor provided in a later stage in the rectifier circuit 805. Note that a limiter circuit may be provided on an input side or an output side of the rectifier circuit 805. The limiter circuit controls electric power so that electric power which is higher than or equal to certain electric power is not input to a circuit in a later stage if the amplitude of the input alternating signal is high and an internal generation voltage is high.

The constant voltage circuit 806 generates a stable power supply voltage from an input potential and supplies it to each circuit. Note that the constant voltage circuit 806 may include a reset signal generation circuit. The reset signal generation circuit is a circuit which generates a reset signal of the logic circuit 809 by utilizing rise of the stable power supply voltage.

The demodulation circuit 807 demodulates the input alternating signal by envelope detection and generates the demodulated signal. Further, the modulation circuit 808 performs modulation in accordance with data to be output from the antenna 804.

The logic circuit 809 analyzes and processes the demodulated signal. The memory circuit 810 holds the input data and includes a row decoder, a column decoder, a memory region, and the like. Further, the ROM 811 stores an identification number (ID) or the like and outputs it in accordance with processing.

Note that decision whether each circuit described above is provided or not can be made as appropriate as needed.

Here, the memory circuit described in the above embodiment can be used as the memory circuit 810. Since the memory circuit of one embodiment of the present invention can retain data even when not powered, the memory circuit can be favorably used for an RFID tag. Furthermore, the memory circuit of one embodiment of the present invention needs power (voltage) needed for data writing significantly lower than that needed in a conventional nonvolatile memory; thus, it is possible to prevent a difference between the maximum communication range in data reading and that in data writing. Furthermore, it is possible to suppress malfunction or incorrect writing which is caused by power shortage in data writing.

Since the memory circuit of one embodiment of the present invention can be used as a nonvolatile memory, it can also be used as the ROM 811. In this case, it is preferable that a manufacturer separately prepare a command for writing data to the ROM 811 so that a user cannot rewrite data freely. Since the manufacturer gives identification numbers before shipment and then starts shipment of products, instead of putting identification numbers to all the manufactured RFID tags, it is possible to put identification numbers to only good products to be shipped. Thus, the identification numbers of the shipped products are in series and customer management corresponding to the shipped products is easily performed.

This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 5

In this embodiment, a CPU that includes the memory device described in the above embodiment is described.

FIG. 10 is a block diagram illustrating a configuration example of a CPU at least partly including any of the transistors described in the above embodiments as a component.

The CPU illustrated in FIG. 10 includes, over a substrate 1190, an arithmetic logic unit (ALU) 1191, an ALU controller 1192, an instruction decoder 1193, an interrupt controller 1194, a timing controller 1195, a register 1196, a register controller 1197, a bus interface 1198, a rewritable ROM 1199, and a ROM interface 1189. A semiconductor substrate, an SOI substrate, a glass substrate, or the like is used as the substrate 1190. The ROM 1199 and the ROM interface 1189 may be provided over a separate chip. Obviously, the CPU illustrated in FIG. 10 is just an example in which the configuration has been simplified, and an actual CPU may have a variety of configurations depending on the application. For example, the CPU may have the following configuration: a structure including the CPU illustrated in FIG. 10 or an arithmetic circuit is considered as one core; a plurality of the cores is included; and the cores operate in parallel. The number of bits that the CPU can process in an internal arithmetic circuit or in a data bus can be 8, 16, 32, or 64, for example.

An instruction that is input to the CPU through the bus interface 1198 is input to the instruction decoder 1193 and decoded therein, and then, input to the ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195.

The ALU controller 1192, the interrupt controller 1194, the register controller 1197, and the timing controller 1195 conduct various controls in accordance with the decoded instruction. Specifically, the ALU controller 1192 generates signals for controlling the operation of the ALU 1191. While the CPU is executing a program, the interrupt controller 1194 processes an interrupt request from an external input/output device or a peripheral circuit depending on its priority or a mask state. The register controller 1197 generates an address of the register 1196, and reads/writes data from/to the register 1196 in accordance with the state of the CPU.

The timing controller 1195 generates signals for controlling operation timings of the ALU 1191, the ALU controller 1192, the instruction decoder 1193, the interrupt controller 1194, and the register controller 1197. For example, the timing controller 1195 includes an internal clock generator for generating an internal clock signal CLK2 on the basis of a reference clock signal CLK1, and supplies the internal clock signal CLK2 to the above circuits.

In the CPU illustrated in FIG. 10, a memory cell is provided in the register 1196. For the memory cell of the register 1196, any of the transistors described in the above embodiments can be used.

In the CPU illustrated in FIG. 10, the register controller 1197 selects operation of holding data in the register 1196 in accordance with an instruction from the ALU 1191. That is, the register controller 1197 selects whether data is held by a flip-flop or by a capacitor in the memory cell included in the register 1196. When data holding by the flip-flop is selected, a power supply voltage is supplied to the memory cell in the register 1196. When data holding by the capacitor is selected, the data is rewritten in the capacitor, and supply of power supply voltage to the memory cell in the register 1196 can be stopped.

FIG. 11 is an example of a circuit diagram of a memory element that can be used as the register 1196. A memory element 1200 includes a circuit 1201 in which stored data is volatile when power supply is stopped, a circuit 1202 in which stored data is nonvolatile even when power supply is stopped, a switch 1203, a switch 1204, a logic element 1206, a capacitor 1207, and a circuit 1220 having a selecting function. The circuit 1202 includes a capacitor 1208, a transistor 1209, and a transistor 1210. Note that the memory element 1200 may further include another element such as a diode, a resistor, or an inductor, as needed.

Here, the memory device described in the above embodiment can be used as the circuit 1202. When supply of a power supply voltage to the memory element 1200 is stopped, a ground potential (0 V) or a potential at which the transistor 1209 in the circuit 1202 is turned off continues to be input to a gate of the transistor 1209. For example, the first gate of the transistor 1209 is grounded through a load such as a resistor.

Shown here is an example in which the switch 1203 is a transistor 1213 having one conductivity type (e.g., an n-channel transistor) and the switch 1204 is a transistor 1214 having a conductivity type opposite to the one conductivity type (e.g., a p-channel transistor). A first terminal of the switch 1203 corresponds to one of a source and a drain of the transistor 1213, a second terminal of the switch 1203 corresponds to the other of the source and the drain of the transistor 1213, and conduction or non-conduction between the first terminal and the second terminal of the switch 1203 (i.e., the on/off state of the transistor 1213) is selected by a control signal RD input to a gate of the transistor 1213. A first terminal of the switch 1204 corresponds to one of a source and a drain of the transistor 1214, a second terminal of the switch 1204 corresponds to the other of the source and the drain of the transistor 1214, and conduction or non-conduction between the first terminal and the second terminal of the switch 1204 (i.e., the on/off state of the transistor 1214) is selected by the control signal RD input to a gate of the transistor 1214.

One of a source and a drain of the transistor 1209 is electrically connected to one of a pair of electrodes of the capacitor 1208 and a gate of the transistor 1210. Here, the connection portion is referred to as a node M2. One of a source and a drain of the transistor 1210 is electrically connected to a line which can supply a low power supply potential (e.g., a GND line), and the other thereof is electrically connected to the first terminal of the switch 1203 (the one of the source and the drain of the transistor 1213). The second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is electrically connected to the first terminal of the switch 1204 (the one of the source and the drain of the transistor 1214). The second terminal of the switch 1204 (the other of the source and the drain of the transistor 1214) is electrically connected to a line which can supply a power supply potential VDD. The second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213), the first terminal of the switch 1204 (the one of the source and the drain of the transistor 1214), an input terminal of the logic element 1206, and one of a pair of electrodes of the capacitor 1207 are electrically connected to each other. Here, the connection portion is referred to as a node M1. The other of the pair of electrodes of the capacitor 1207 can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor 1207 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor 1207 is electrically connected to the line which can supply a low power supply potential (e.g., a GND line). The other of the pair of electrodes of the capacitor 1208 can be supplied with a constant potential. For example, the other of the pair of electrodes of the capacitor 1208 can be supplied with a low power supply potential (e.g., GND) or a high power supply potential (e.g., VDD). The other of the pair of electrodes of the capacitor 1208 is electrically connected to the line which can supply a low power supply potential (e.g., a GND line).

The capacitor 1207 and the capacitor 1208 are not necessarily provided as long as the parasitic capacitance of the transistor, the wiring, or the like is actively utilized.

A control signal WE is input to the first gate (first gate electrode) of the transistor 1209. As for each of the switch 1203 and the switch 1204, a conduction state or a non-conduction state between the first terminal and the second terminal is selected by the control signal RD which is different from the control signal WE. When the first terminal and the second terminal of one of the switches are in the conduction state, the first terminal and the second terminal of the other of the switches are in the non-conduction state.

Note that the transistor 1209 in FIG. 11 has a structure with a second gate (second gate electrode: back gate). The control signal WE can be input to the first gate and a control signal WE2 can be input to the second gate. The control signal WE2 is a signal having a constant potential. As the constant potential, for example, a ground potential GND or a potential lower than a source potential of the transistor 1209 is selected. The control signal WE2 is a potential signal for controlling the threshold voltage of the transistor 1209, and a drain current of the transistor 1209 at a gate voltage of 0 V can be further reduced. The control signal WE2 may be a signal having the same potential as that of the control signal WE. Note that as the transistor 1209, a transistor without a second gate may be used.

A signal corresponding to data retained in the circuit 1201 is input to the other of the source and the drain of the transistor 1209. FIG. 11 illustrates an example in which a signal output from the circuit 1201 is input to the other of the source and the drain of the transistor 1209. The logic value of a signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is inverted by the logic element 1206, and the inverted signal is input to the circuit 1201 through the circuit 1220.

In the example of FIG. 11, a signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) is input to the circuit 1201 through the logic element 1206 and the circuit 1220; however, one embodiment of the present invention is not limited thereto. The signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) may be input to the circuit 1201 without its logic value being inverted. For example, in the case where the circuit 1201 includes a node in which a signal obtained by inversion of the logic value of a signal input from the input terminal is retained, the signal output from the second terminal of the switch 1203 (the other of the source and the drain of the transistor 1213) can be input to the node.

In FIG. 11, the transistors included in the memory element 1200 except for the transistor 1209 can each be a transistor in which a channel is formed in a layer formed using a semiconductor other than an oxide semiconductor or in the substrate 1190. For example, the transistor can be a transistor in which a channel is formed in a silicon layer or a silicon substrate. Alternatively, all the transistors in the memory element 1200 may be a transistor in which a channel is formed in an oxide semiconductor. Further alternatively, in the memory element 1200, a transistor in which a channel is formed in an oxide semiconductor can be included besides the transistor 1209, and a transistor in which a channel is formed in a layer or the substrate 1190 including a semiconductor other than an oxide semiconductor can be used for the rest of the transistors.

As the circuit 1201 in FIG. 11, for example, a flip-flop circuit can be used. As the logic element 1206, for example, an inverter or a clocked inverter can be used.

In a period during which the memory element 1200 is not supplied with the power supply voltage, the semiconductor device of one embodiment of the present invention can retain data stored in the circuit 1201 by the capacitor 1208 which is provided in the circuit 1202.

The off-state current of a transistor in which a channel is formed in an oxide semiconductor is extremely low. For example, the off-state current of a transistor in which a channel is formed in an oxide semiconductor is significantly lower than that of a transistor in which a channel is formed in silicon having crystallinity. Thus, when the transistor is used as the transistor 1209, a signal held in the capacitor 1208 is retained for a long time also in a period during which the power supply voltage is not supplied to the memory element 1200. The memory element 1200 can accordingly retain the stored content (data) also in a period during which the supply of the power supply voltage is stopped.

Since the above-described memory element performs pre-charge operation with the switch 1203 and the switch 1204, the time required for the circuit 1201 to retain original data again after the supply of the power supply voltage is restarted can be shortened.

In the circuit 1202, a signal retained by the capacitor 1208 is input to the gate of the transistor 1210. Therefore, after supply of the power supply voltage to the memory element 1200 is restarted, the signal retained by the capacitor 1208 can be converted into the one corresponding to the state (the on state or the off state) of the transistor 1210 to be read from the circuit 1202. Consequently, an original signal can be accurately read even when a potential corresponding to the signal retained by the capacitor 1208 varies to some degree.

By applying the above-described memory element 1200 to a memory device such as a register or a cache memory included in a processor, data in the memory device can be prevented from being lost owing to the stop of the supply of the power supply voltage. Further, shortly after the supply of the power supply voltage is restarted, the memory element can be returned to the same state as that before the power supply is stopped. Therefore, the power supply can be stopped even for a short time in the processor or one or a plurality of logic circuits included in the processor. Accordingly, power consumption can be suppressed.

Although the memory element 1200 is used in a CPU in this embodiment, the memory element 1200 can also be used in an LSI such as a digital signal processor (DSP), a custom LSI, or a programmable logic device (PLD), and a radio frequency identification (RF-ID).

This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 6

In this embodiment, configuration examples of a display device using a transistor of one embodiment of the present invention are described.

<Structure Example>

FIG. 12A is a top view of the display device of one embodiment of the present invention. FIG. 12B is a circuit diagram illustrating a pixel circuit that can be used in the case where a liquid crystal element is used in a pixel in the display device of one embodiment of the present invention. FIG. 12C is a circuit diagram illustrating a pixel circuit that can be used in the case where an organic EL element is used in a pixel in the display device of one embodiment of the present invention.

The transistor in the pixel portion can be formed in accordance with the above embodiments. Further, the transistor can easily be an n-channel transistor, and thus, part of a driver circuit that can be formed using an n-channel transistor in the driver circuit is formed over the same substrate as the transistor of the pixel portion. By using the transistor described in the above embodiment for the pixel portion or the driver circuit as described above, a highly reliable display device can be provided.

FIG. 12A illustrates an example of a top view of an active matrix display device. A pixel portion 701, a first scan line driver circuit 702, a second scan line driver circuit 703, and a signal line driver circuit 704 are formed over a substrate 700 of the display device. In the pixel portion 701, a plurality of signal lines extended from the signal line driver circuit 704 are arranged and a plurality of scan lines extended from the first scan line driver circuit 702 and the second scan line driver circuit 703 are arranged. Note that pixels each including a display element are provided in matrix in respective regions in each of which the scan line and the signal line intersect with each other. The substrate 700 of the display device is connected to a timing control circuit (also referred to as a controller or a controller IC) through a connection portion such as a flexible printed circuit (FPC).

In FIG. 12A, the first scan line driver circuit 702, the second scan line driver circuit 703, and the signal line driver circuit 704 are formed over the substrate 700 where the pixel portion 701 is formed. Accordingly, the number of components of a driver circuit which is provided outside and the like are reduced, so that a reduction in cost can be achieved. Furthermore, if the driver circuit is provided outside the substrate 700, wirings would need to be extended and the number of wiring connections would increase. When the driver circuit is provided over the substrate 700, the number of connections of the wirings can be reduced. Consequently, an improvement in reliability or yield can be achieved. One or more of the first scan line driver circuit 702, the second scan line driver circuit 703, and the signal line driver circuit 704 may be mounted on the substrate 700 or provided outside the substrate 700.

<Liquid Crystal Display Device>

FIG. 12B illustrates an example of a circuit configuration of the pixel. Here, a pixel circuit which is applicable to a pixel of a VA liquid crystal display device is illustrated as an example.

This pixel circuit can be applied to a structure in which one pixel includes a plurality of pixel electrode layers. The pixel electrode layers are connected to different transistors, and the transistors can be driven with different gate signals. Accordingly, signals applied to individual pixel electrode layers in a multi-domain pixel can be controlled independently.

A gate wiring 712 of a transistor 716 and a gate wiring 713 of a transistor 717 are separated so that different gate signals can be supplied thereto. In contrast, a data line 714 is shared by the transistors 716 and 717. Any of the transistors described in the above embodiments can be used as appropriate as each of the transistors 716 and 717. Thus, a highly reliable liquid crystal display device can be provided.

A first pixel electrode layer is electrically connected to the transistor 716 and a second pixel electrode layer is electrically connected to the transistor 717. The first pixel electrode layer and the second pixel electrode layer are separated. Shapes of the first pixel electrode layer and the second pixel electrode layer are not especially limited. For example, the first pixel electrode layer may have a V-like shape.

A gate electrode of the transistor 716 is connected to the gate wiring 712, and a gate electrode of the transistor 717 is connected to the gate wiring 713. When different gate signals are supplied to the gate wiring 712 and the gate wiring 713, operation timings of the transistor 716 and the transistor 717 can be varied. As a result, alignment of liquid crystals can be controlled.

Further, a storage capacitor may be formed using a capacitor wiring 710, a gate insulating film functioning as a dielectric, and a capacitor electrode electrically connected to the first pixel electrode layer or the second pixel electrode layer.

The multi-domain pixel includes a first liquid crystal element 718 and a second liquid crystal element 719. The first liquid crystal element 718 includes the first pixel electrode layer, a counter electrode layer, and a liquid crystal layer therebetween. The second liquid crystal element 719 includes the second pixel electrode layer, a counter electrode layer, and a liquid crystal layer therebetween.

Note that a pixel circuit of the present invention is not limited to that shown in FIG. 12B. For example, a switch, a resistor, a capacitor, a transistor, a sensor, a logic circuit, or the like may be added to the pixel circuit illustrated in FIG. 12B.

<Organic EL Display Device>

FIG. 12C illustrates another example of a circuit configuration of the pixel. Here, a pixel structure of a display device using an organic EL element is shown.

In an organic EL element, by application of voltage to a light-emitting element, electrons are injected from one of a pair of electrodes and holes are injected from the other of the pair of electrodes, into a layer containing a light-emitting organic compound; thus, current flows. The electrons and holes are recombined, and thus, the light-emitting organic compound is excited. The light-emitting organic compound returns to a ground state from the excited state, thereby emitting light. Based on such a mechanism, such a light-emitting element is referred to as a current-excitation type light-emitting element.

FIG. 12C illustrates an applicable example of a pixel circuit. In this example, one pixel includes two n-channel transistors. Further, digital time grayscale driving can be employed for the pixel circuit.

The configuration of the applicable pixel circuit and operation of a pixel employing digital time grayscale driving will be described.

A pixel 720 includes a switching transistor 721, a driver transistor 722, a light-emitting element 724, and a capacitor 723. A gate electrode layer of the switching transistor 721 is connected to a scan line 726, a first electrode (one of a source electrode layer and a drain electrode layer) of the switching transistor 721 is connected to a signal line 725, and a second electrode (the other of the source electrode layer and the drain electrode layer) of the switching transistor 721 is connected to a gate electrode layer of the driver transistor 722. The gate electrode layer of the driver transistor 722 is connected to a power supply line 727 through the capacitor 723, a first electrode of the driver transistor 722 is connected to the power supply line 727, and a second electrode of the driver transistor 722 is connected to a first electrode (a pixel electrode) of the light-emitting element 724. A second electrode of the light-emitting element 724 corresponds to a common electrode 728. The common electrode 728 is electrically connected to a common potential line formed over the same substrate as the common electrode 728.

As the switching transistor 721 and the driver transistor 722, any of the transistors described in other embodiments can be used as appropriate. In this manner, a highly reliable organic EL display device can be provided.

The potential of the second electrode (the common electrode 728) of the light-emitting element 724 is set to be a low power supply potential. Note that the low power supply potential is lower than a high power supply potential supplied to the power supply line 727. For example, the low power supply potential can be GND, 0 V, or the like. The high power supply potential and the low power supply potential are set to be higher than or equal to the forward threshold voltage of the light-emitting element 724, and the difference between the potentials is applied to the light-emitting element 724, whereby current is supplied to the light-emitting element 724, leading to light emission. The forward voltage of the light-emitting element 724 refers to a voltage at which a desired luminance is obtained, and includes at least a forward threshold voltage.

Note that gate capacitance of the driver transistor 722 may be used as a substitute for the capacitor 723, so that the capacitor 723 can be omitted.

Next, a signal input to the driver transistor 722 will be described. In the case of a voltage-input voltage driving method, a video signal for sufficiently turning on or off the driver transistor 722 is input to the driver transistor 722. In order for the driver transistor 722 to operate in a linear region, voltage higher than the voltage of the power supply line 727 is applied to the gate electrode layer of the driver transistor 722. Note that voltage higher than or equal to voltage which is the sum of power supply line voltage and the threshold voltage Vth of the driver transistor 722 is applied to the signal line 725.

In the case of performing analog grayscale driving, a voltage greater than or equal to a voltage which is the sum of the forward voltage of the light-emitting element 724 and the threshold voltage Vth of the driver transistor 722 is applied to the gate electrode layer of the driver transistor 722. A video signal by which the driver transistor 722 is operated in a saturation region is input, so that current is supplied to the light-emitting element 724. In order for the driver transistor 722 to operate in a saturation region, the potential of the power supply line 727 is set higher than the gate potential of the driver transistor 722. When an analog video signal is used, it is possible to supply current to the light-emitting element 724 in accordance with the video signal and perform analog grayscale driving.

Note that the configuration of the pixel circuit of the present invention is not limited to that shown in FIG. 12C. For example, a switch, a resistor, a capacitor, a sensor, a transistor, a logic circuit, or the like may be added to the pixel circuit shown in FIG. 12C.

In the case where the transistor shown in any of the above embodiments is used for the circuit shown in FIGS. 12A to 12C, the source electrode (the first electrode) is electrically connected to the low potential side and the drain electrode (the second electrode) is electrically connected to the high potential side. Furthermore, the potential of the first gate electrode may be controlled by a control circuit or the like and the potential described above as an example, e.g., a potential lower than the potential applied to the source electrode, may be input to the second gate electrode through a wiring that is not illustrated.

In this specification and the like, for example, a display element, a display device which is a device including a display element, a light-emitting element, and a light-emitting device which is a device including a light-emitting element can employ a variety of modes or can include a variety of elements. The display element, the display device, the light-emitting element, or the light-emitting device includes at least one of an electroluminescence (EL) element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), an electron emitter, a liquid crystal element, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a display element using micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), an interferometric modulator display (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, an electrowetting element, a piezoelectric ceramic display, a display element including a carbon nanotube, and the like. Other than the above, a display medium whose contrast, luminance, reflectance, transmittance, or the like is changed by electrical or magnetic action may be included. Note that examples of a display device including an EL element include an EL display. Examples of a display device including an electron emitter include a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of a display device including a liquid crystal element include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). Examples of a display device including electronic ink or an electrophoretic element include electronic paper. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to contain aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption.

This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 7

In this embodiment, a display module using a semiconductor device of one embodiment of the present invention is described with reference to FIG. 13.

In a display module 8000 in FIG. 13, a touch panel 8004 connected to an FPC 8003, a display panel 8006 connected to an FPC 8005, a backlight unit 8007, a frame 8009, a printed board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002. Note that the backlight unit 8007, the battery 8011, the touch panel 8004, and the like are not provided in some cases.

The semiconductor device of one embodiment of the present invention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch panel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitive touch panel and may be formed so as to overlap with the display panel 8006. A counter substrate (sealing substrate) of the display panel 8006 can have a touch panel function. A photosensor may be provided in each pixel of the display panel 8006 so that an optical touch panel function is added. An electrode for a touch sensor may be provided in each pixel of the display panel 8006 so that a capacitive touch panel function is added. A display module with a position input function may be used as the display panel 8006. Note that the position input function can be added by providing the display panel 8006 with the touch panel 8004.

The backlight unit 8007 includes a light source 8008. The light source 8008 may be provided at an end portion of the backlight unit 8007 and a light diffusing plate may be used.

The frame 8009 protects the display panel 8006 and also functions as an electromagnetic shield for blocking electromagnetic waves generated from the printed board 8010. The frame 8009 can function as a radiator plate.

The printed board 8010 has a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or the battery 8011 provided separately may be used. Note that the battery 8011 is not necessary in the case where a commercial power source is used.

The display module 8000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

Embodiment 8

The semiconductor device of one embodiment of the present invention can be used for display devices, personal computers, or image reproducing devices provided with recording media (typically, devices which reproduce the content of recording media such as digital versatile discs (DVDs) and have displays for displaying the reproduced images). Other examples of electronic devices that can be equipped with the semiconductor device of one embodiment of the present invention are mobile phones, game machines including portable game consoles, portable data terminals, e-book readers, cameras such as video cameras and digital still cameras, goggle-type displays (head mounted displays), navigation systems, audio reproducing devices (e.g., car audio systems and digital audio players), copiers, facsimiles, printers, multifunction printers, automated teller machines (ATM), and vending machines. FIGS. 14A to 14F illustrate specific examples of these electronic devices.

FIG. 14A illustrates a portable game console including a housing 901, a housing 902, a display portion 903, a display portion 904, a microphone 905, a speaker 906, an operation key 907, a stylus 908, and the like. Although the portable game console in FIG. 14A has the two display portions 903 and 904, the number of display portions included in a portable game console is not limited to this.

FIG. 14B illustrates a portable data terminal including a first housing 911, a second housing 912, a first display portion 913, a second display portion 914, a joint 915, an operation key 916, and the like. The first display portion 913 is provided in the first housing 911, and the second display portion 914 is provided in the second housing 912. The first housing 911 and the second housing 912 are connected to each other with the joint 915, and the angle between the first housing 911 and the second housing 912 can be changed with the joint 915. An image on the first display portion 913 may be switched depending on the angle between the first housing 911 and the second housing 912 at the joint 915. A display device with a position input function may be used as at least one of the first display portion 913 and the second display portion 914. Note that the position input function can be added by providing a touch panel in a display device. Alternatively, the position input function can be added by provision of a photoelectric conversion element called a photosensor in a pixel portion of a display device.

FIG. 14C illustrates a notebook personal computer, which includes a housing 921, a display portion 922, a keyboard 923, a pointing device 924, and the like.

FIG. 14D illustrates a wrist-watch-type information terminal, which includes a housing 931, a display portion 932, a wristband 933, and the like. The display portion 932 may be a touch panel.

FIG. 14E illustrates a video camera, which includes a first housing 941, a second housing 942, a display portion 943, operation keys 944, a lens 945, a joint 946, and the like. The operation keys 944 and the lens 945 are provided for the first housing 941, and the display portion 943 is provided for the second housing 942. The first housing 941 and the second housing 942 are connected to each other with the joint 946, and the angle between the first housing 941 and the second housing 942 can be changed with the joint 946. Images displayed on the display portion 943 may be switched in accordance with the angle at the joint 946 between the first housing 941 and the second housing 942.

FIG. 14F illustrates an ordinary vehicle including a car body 951, wheels 952, a dashboard 953, lights 954, and the like.

Note that this embodiment can be combined as appropriate with any of the other embodiments and examples in this specification.

Embodiment 9

In this embodiment, application examples of an RFID of one embodiment of the present invention are described with reference to FIGS. 15A to 15F. The RFID is widely used and can be provided for, for example, products such as bills, coins, securities, bearer bonds, documents (e.g., driver's licenses or resident cards, see FIG. 15A), recording media (e.g., DVDs or video tapes, see FIG. 15B), vehicles (bicycles or the like, see FIG. 15C), packaging containers (e.g., wrapping paper or bottles, see FIG. 15D), foods, plants, animals, human bodies, clothes, household goods, medical supplies such as medicine and chemicals, and electronic devices (e.g., liquid crystal display devices, EL display devices, television devices, or mobile phones), or tags on products (see FIGS. 15E and 15F).

An RFID 4000 of one embodiment of the present invention is fixed to a product by being attached to a surface thereof or embedded therein. For example, the RFID 4000 is fixed to each product by being embedded in paper of a book, or embedded in an organic resin of a package. Since the RFID 4000 of one embodiment of the present invention can be reduced in size, thickness, and weight, it can be fixed to a product without spoiling the design of the product. Further, bills, coins, securities, bearer bonds, documents, or the like can have an identification function by being provided with the RFID 4000 of one embodiment of the present invention, and the identification function can be utilized to prevent counterfeiting. Moreover, the efficiency of a system such as an inspection system can be improved by providing the RFID of one embodiment of the present invention for packaging containers, recording media, personal belongings, foods, clothing, household goods, electronic devices, or the like. Vehicles can also have higher security against theft or the like by being provided with the RFID of one embodiment of the present invention.

As described above, by using the RFID of one embodiment of the present invention for each application described in this embodiment, power for operation such as writing or reading of data can be reduced, which results in an increase in the maximum communication distance. Moreover, data can be held for an extremely long period even in the state where power is not supplied; thus, the RFID can be preferably used for application in which data is not frequently written or read.

This embodiment can be combined as appropriate with any of the other embodiments and an example in this specification.

This application is based on Japanese Patent Application serial no. 2014-096190 filed with Japan Patent Office on May 7, 2014, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: an oxide semiconductor; an insulating film; a gate insulating film; and a gate electrode, wherein the oxide semiconductor includes a first portion and a second portion over the first portion, wherein the insulating film includes a region in contact with a side surface of the first portion, and wherein the gate electrode includes a region that covers the second portion with the gate insulating film provided therebetween.
 2. The semiconductor device according to claim 1, wherein the second portion is above the insulating film in a thickness direction of the insulating film.
 3. The semiconductor device according to claim 1, wherein the oxide semiconductor contains at least one element selected from indium (In), gallium (Ga), and zinc (Zn).
 4. The semiconductor device according to claim 1, wherein the insulating film has a function of releasing oxygen by heating.
 5. An electronic device comprising the semiconductor device according to claim 1 and a display device.
 6. An electronic device comprising the semiconductor device according to claim 1 and a battery. 